type-bounds.scala

class A
class A2 extends A
class B

trait M[X]

//
// Upper Type Bound
//
def upperTypeBound[AA <: A](x: AA): A = x

// equivalently, require a implicit parameter of <:<[AA, A].
// This technique must be used if A is an abstract type from the
// enclosing scope.
def upperTypeBound2[AA](x: AA)(implicit ev: AA <:< A): A = x // compiles to ev(x)


//
// Lower Type Bound
//
def lowerTypeBound[AA >: A](x: AA, pred: (AA => Boolean)): Boolean = pred(x)

// equivalently, require a implicit parameter of <:<[A, AA].
// This technique must be used if A is an abstract type from the
// enclosing scope.
def lowerTypeBound2[AA >: A](x: AA, pred: (AA => Boolean))(implicit ev: A <:< AA): Boolean = pred(x)


//
// View Bound
//
def viewBound[AA <% A](x: AA): A = x

//compiles to:
def viewBound$[AA](x: AA)(implicit ev1$ : AA => A) = ev1$(x)


//
// Context Bound
//
def contextBound[X: M](x: X) = x;

//compiles to:
def contextBound$[X](x: X)(implicit ev1$ : M[X]) = x

//
// Usage
//
{
  {

    upperTypeBound(new A)
    upperTypeBound(new A2)
  }

  {
    val predA = (a: A) => true
    val predAny = (a: Any) => true
    lowerTypeBound(new A, predA)
    lowerTypeBound(new {}, predAny)
  }

  {
    implicit def String2A(s: String): A = new A
    viewBound("")
  }
  {
    implicit def MofA: M[A] = new M[A] {}
    contextBound(new A)
  }
}